Grab a *.elf file ( HackMii Installer's boot.elf works) and put that in the root of the SD card.
Smash Stack will not work if the console has any custom stages on it, even the three samples that come with a new file or the stages from the Smash Service. You must have an SD card that is 2GB or less. Note: An SDHC card will not work with Super Smash Brothers Brawl.